Digital Circuits Having Improved Transistors, and Methods Therefor

ABSTRACT

Digital circuits are disclosed that may include multiple transistors having controllable current paths coupled between first and second logic nodes. One or more of the transistors may have a deeply depleted channel formed below its gate that includes a substantially undoped channel region formed over a relatively highly doped screen layer formed over a doped body region. Resulting reductions in threshold voltage variation may improve digital circuit performance. Logic circuit, static random access memory (SRAM) cell, and passgate embodiments are disclosed.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. application Ser.No. 13/030,939 which is hereby incorporated herein by reference.

TECHNICAL FIELD

The present invention relates generally to electronic circuits, and moreparticularly to digital circuits for processing and/or storing digitalvalues.

BACKGROUND

Electronic circuits, typically incorporated within integrated circuit(IC) devices, determine the function of various electronic systems,ranging from large systems (such as computer server “farms” that enablethe wide variety of Internet services and businesses, and can includehundreds or even thousands of server computers), to the small portableelectronic devices such as cellular telephones.

Electronic circuits typically include transistors interconnected to oneanother to a same integrated circuit substrate and/or package. Anintegrated circuit (IC) substrate may be a single semiconductorsubstrate (e.g., die created by dividing a fabricated “wafer”) thatincludes circuit elements of the electronic circuit. An integratedcircuit package may present a set of external connections, but includeone or more ICs substrates and circuit components having conductiveinterconnections to one another.

Digital electronic circuits (hereinafter digital circuits) may form allor a portion of a large majority of circuits included within ICs.Digital circuits may receive and output digital values, typically binaryvalues that vary between low and high logic levels.

Continuing goals for circuits (including digital circuits) includereductions in power consumption, improvements in performance, andreductions in area occupied by the circuit. Because ICs may employ vastnumbers (up to millions) of digital circuits, even incrementalreductions in power consumption may translate into significant powersavings of devices or systems employing such circuits. In the case oflarge systems, reductions in power consumption can reduce power costs ofan enterprise. In the case of portable electronic devices, reductions inpower consumption can advantageously lead to longer battery life and/orthe ability to provide additional functions for a given amount ofcharge.

Performance may include various aspects of circuit operation, includingbut not limited to: the speed at which data values transmitted and/oraccessed by digital circuits. Improvements in signal propagation time(e.g., speed) may enable a device to increase the speed at which data istransmitted between locations of a device, thus reducing the time forthe device to execute operations. In devices where data is stored, thespeed at which data is written and/or read from storage locations maylikewise improve device performance. Performance may also includecircuit stability. Stability may be the ability of a circuit to providea sufficient response under particular operating conditions.

Reductions in circuit size may directly translate into cost savings. Inthe case of ICs, reductions in size may allow more devices to fit on afabrication substrate: As understood from above, digital circuits mayoccupy substantially all of the substrate area for some devices, andsignificant amount of are for others.

As device fabrication technologies approach limits to scaling (i.e., theability to reduce circuit element sizes) the ability to further advanceany of the goals noted above has grown increasingly costly ortechnically challenging.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an integrated circuit device according to an embodiment.

FIG. 2A shows a deeply depleted channel (DOC) transistor that may beincluded in embodiments.

FIG. 2B shows a conventional transistor.

FIGS. 3A and 3B show inverter circuits according to embodiments.

FIGS. 4A and 4B show NAND and NOR logic circuits according toembodiments.

FIG. 4C is a graph illustrating simulation results of a thresholdvoltage (Vt) of a DOC device at various bias voltages.

FIG. 5 shows a digital circuit according to an embodiment that includesserially connected digital stages.

FIG. 6 is a table showing signal propagation simulation results for aclock tree according to an embodiment and according to a conventionalapproach.

FIG. 7 shows a passgate circuit according to an embodiment.

FIG. 8 shows a flip-flop circuit according to an embodiment.

FIG. 9 shows a dynamic logic circuit according to an embodiment.

FIGS. 10A and 10B show one of many logic sections that can be includedin the embodiment of FIG. 9.

FIG. 11 shows a current steering logic circuit according to anembodiment.

FIGS. 12A and 12B show one of many logic sections that can be includedin the embodiment of FIG. 11.

FIG. 13 shows a latch circuit according to an embodiment.

FIG. 14 shows a six transistor (6-T) static random access memory (SRAM)cell according to an embodiment.

FIGS. 15A and 15B show simulation conditions and results for a 6-T SRAMcell according to one embodiment.

FIGS. 16A and 16B show simulation conditions and results for aconventional 6-T SRAM cell.

FIG. 17 shows a DOC transistor that may be included in embodiments.

FIG. 18 shows another DOC transistor that may be included inembodiments.

FIG. 19 is a top plan view of an IC device according to an embodimenthaving both DOC transistors and non-DOC transistors.

DETAILED DESCRIPTION

Various embodiments of the present invention will now be described indetail with reference to a number of drawings. The embodiments showdigital circuits and related methods that may be included in integratedcircuit devices to provide improved performance over conventionaldigital circuit approaches.

In the various embodiments below, like items are referred to by the samereference character but the leading digits corresponding to the figurenumber.

Referring now to FIG. 1 an integrated circuit (IC) device according toone embodiment is show in a top plan view, and designated by the generalreference character 100. An IC device 100 may be formed as a “die”having substrate 101 containing the various circuits therein. An ICdevice 100 may include one or more circuit sections, and FIG. 1identifies four circuit sections as 102-0 to 102-3. Any or all ofcircuit sections (102-0 to 102-3) may include digital circuit blocksthat perform digital functions for the IC device 100.

In the embodiment shown, circuit section 102-2 may be a digital circuitblock that includes one or more digital circuits, one shown as 104. Adigital circuit 104 may generate output signals on one or more outputnodes (e.g., 108) in response to input signals received on one or moreinput nodes (e.g., 110). It is noted that in some embodiments, an outputnode and input node may be the same node.

Referring still to FIG. 1, a digital circuit 104 may include one or more“deeply depleted channel” (DDC) transistors. A DDC transistor includesboth a highly doped “screening” layer below a gate that defines theextent of the depletion region below the gate in operation, and anundoped channel extending between source and drain of a transistor.Typically, to prevent contamination of the undoped channel, transistorsare manufactured without halo or “pocket” implants, and annealconditions are tightly controlled to prevent unwanted diffusion ofdopants into the undoped channel. While conventional threshold voltage(Vt) implants are also avoided to prevent channel contamination, Vt setlayers that are grown as blanket or selective epitaxial layers on thescreening layer can be used to finely adjust or tune the thresholdvoltage of individual or blocks of transistors. Further examples of DDCtransistor structure and manufacture are disclosed in U.S. patentapplication Ser. No. 12/708,497, filed on

February 18, 2010, titled ELECTRONIC DEVICES AND SYSTEMS, AND METHODSFOR MAKING AND USING THE SAME, by Scott E. Thompson et al., as well asU.S. patent application Ser. No. 12/971,884, filed on Dec. 17, 2010titled LOW POWER SEMICONDUCTOR TRANSISTOR STRUCTURE AND METHOD OFFABRICATION THEREOF and U.S. patent application Ser. No. 12/971,955filed on Dec. 17, 2010 titled TRANSISTOR WITH THRESHOLD VOLTAGE SETNOTCH AND METHOD OF FABRICATION THEREOF the respective contents of whichare incorporated by reference herein.

DDC transistors included within a digital circuit may include n-channeltransistors, p-channel transistors or both. N-channel DDC transistorswill be represented in this disclosure by the symbol shown as 106-0 inFIG. 1. P-channel DDC transistors will be represented in this disclosureby the symbol shown as 106-1 in FIG. 1. DDC transistors mayadvantageously include a substantially undoped channel region formedover a relatively highly doped screening layer.

Specifically referring now to FIG. 2A, one exemplary representation of aDDC transistor is shown in a side cross sectional view, and designatedby the general reference character 206. DDC transistor 206 may include agate 212 separated from a substrate 224 by a gate insulator 222. Asubstantially undoped channel region 214 may be formed below gate 212. Adoped screening layer 216 may be formed below channel region 214. It isunderstood that there may be other layers between channel region 214 andscreening layer 216. A substrate 224 may be formed of more than onesemiconductor layer. As but one example, a substrate may include one ormore “epitaxial” layers formed on a bulk semiconductor substrate.

A screening layer 216 may be doped to an opposite conductivity type ofthe transistor channel type (e.g., an n-channel DDC transistor will havea p-doped screening layer). A screening layer 216 doping concentrationmay be greater than a concentration of a body region 218. FIG. 2A alsoshows source and drain regions 220 on opposing lateral sides of channelregion 214. Source and drain regions 220 may include a source and draindiffusion. More particular types of DDC source and drain structures,relative to substantially undoped channel region will be described inmore detail below.

Referring to FIG. 2B, one representation of a conventional transistor isshown for comparison to that shown in FIG. 2A. Conventional transistor205 may include a gate 213 separated from a substrate 225 by a gateinsulator 223. A channel region 215 may be formed below a gate 213between source/drain diffusions 221. A channel region 215 may be dopedto a conductivity type opposite to that of source/drain diffusions 221,and the same as that of a transistor body region 219.

In this way, a digital circuit may be formed with one or more DDCtransistors.

Referring now to FIG. 3A one example of a digital circuit according toan embodiment is shown in a schematic diagram, and designated by thegeneral reference character 304. Digital circuit 304 may be an inverterthat inverts an input signal received on an input node 310 to generatean output signal OUT on an output node 308. Digital circuit 304 mayinclude a pull-down transistor 326-0 and a pull-up transistor 326-1 that30 drive output node 308 between a high logic level (e.g., VHI) and alow logic level (e.g., VLO). In the embodiment shown, pull-up transistor326-1 may be an re-channel DDC transistor, while pull-up transistor326-1 may optionally be a p-channel DDC transistor (as indicated byhashing). However, in alternate embodiments all or anyone of thetransistors may be a DDC transistor. It is noted that FIG. 3A shows a“static” logic embodiment where a signal generated on output node 308may have a value, a timing established by input signal IN.

In the embodiment of FIG. 3A, transistors 326-0/1 have source tied“bodies”. Thus, any screening region of a transistor may be driven withlogic supply voltage (Le., a logic high or low voltage shown as VHI andVLO).

FIG. 3B shows a digital circuit 304′ having a configuration with similarconnections to those of FIG. 3A. However, unlike FIG. 3A, transistors(326-2/3) of FIG. 3B may have body bias voltages different thancorresponding drain voltages. Thus, a screening region of any suchindependent body transistor may be driven with a body bias voltage(shown as Pbias, Nbias) that may be different than drain voltages. Abody bias voltage may be static or dynamic. In embodiments having DDCtransistors of both conductivity types, one or both types of transistorsmay receive a body bias voltage. In embodiments having a DDC transistorand non-DDC transistor, one or both types of transistors may receive abody bias voltage.

In this way, an inverter may include one or more DDC transistors with orwithout separately biased bodies.

Referring now to FIGS. 4A and 4B, additional digital circuits accordingto embodiments are shown in block schematic diagrams, and designated bythe general reference characters 404 and 404′, respectively. Digitalcircuit 404/404′ may include multiple transistors (426-0 to -3)interconnected to one another between a high logic node VHI and lowlogic node VLO. The transistor types and body connections may vary asnoted for FIGS. 3A and 3B. In particular, any number (including all)transistors may be DDC transistors. Further, any number (including all)transistors (regardless of whether they DDC transistors or not), mayhave bodies connected to a logic high or low voltage, or may have bodiesbiased with a different voltage (such a body bias voltage being staticor dynamic). FIG. 4A shows a static logic NAND gate, while FIG. 4B showsa static logic NOR gate. From these logic gates one skilled in the artcould arrive at more complex logic circuits that may include DDC typetransistors.

Digital circuits employing DDC transistors as described herein, andequivalents, may provide a wider range of performance modulation thanconventional approaches. As noted above, in various logic circuits shownherein, bodies of DDC transistors may be biased with a voltage otherthan a logic high or logic low voltage. Such body biasing of DDCtransistors may provide for greater variation in transistor thresholdvoltage per applied body bias, as compared to doped channel devices. Onevery particular example of such bias variation is shown in FIG. 4C.

FIG. 4C is a graph showing simulation results of a threshold voltage(Vt) of a DDC device at various bias voltages. Curve 400 representsresults corresponding to a 500 nanometer (nm) gate length DDCtransistor. Curve 402 shows a conventional 500 nm transistor response.It is understood that FIG. 4C is but one response, and such biasingresponse may vary according to device geometry, doping, screeningchannel position, any Vt adjustment layer, gate insulator thickness,gate material, to name but a few examples.

Digital circuits employing DDC transistor as described herein, may varyoperations with body biasing. For example, higher body bias may beutilized to reduce power in standby states. Such body biasing may alsovary with changes in logic levels, which may be a power supply voltagein some embodiments (e.g., VHI=VDD, VLO=VSS).

Referring now to FIG. 5, a digital circuit according to a furtherembodiment is shown in a block schematic diagram, and designated by thegeneral reference character 504. A digital circuit 504 may include aseries of stages 504-0 to -M, where each stage may be a logic circuit asdescribed herein or an equivalent. One or more input signals (IN0 toINj) may be logically operated on, to propagate signals through stages(504-0 to -M) and generate one or more output stages OUT0 to OUTk. Thatis, each of stages (504-0 to -M) may drive nodes within between logichigh (VHI) and logic low (VLO) levels.

Such digital circuits that include stages with DDC transistors may havevarious advantageous features over conventional digital circuits aswould be understood by those skilled in the art in light of the variousdiscussions herein. However, in particular embodiments, digital circuitsthat employ DDC devices may have less variation in response.Consequently, when such digital circuits include stages, as shown inFIG. 5, an average expected signal propagation time may exhibit lessvariation, and hence be faster.

FIG. 6 is a table showing simulation data comparing digital circuitsaccording embodiments (DDC DEVICES) versus a corresponding conventionalapproach. The digital circuits of FIG. 6 may be signal distributionchains composed of a number of stages (60, 30, 10), where each stageincludes an inverter. That is, the DDC DEVICES may be one implementationof that shown in FIG. 5, where each stage includes an inverter like thatof FIG. 2A.

FIG. 6 shows results for high and low simulation temperatures (i.e., 0°C. and 85° C.) for signal chains of 60 stages, 30 stages and 10 stages.Results are given as 3σ distributions. As shown, distributions may betighter in the DDC DEVICES cases. Such tighter distributions may allowfor faster signal distribution and/or processing environments, improvinga device or system performance.

In this way, digital circuits having series connected stages may includeDDC devices to reduce signal propagation time.

While embodiments may include DDC transistors that drive nodes betweenhigh and/or a low logic levels, other embodiments may serve to passsignals from one node to another. One such embodiment is shown in FIG.7.

Referring now to FIG. 7 one example of a passgate circuit according toan embodiment is shown in a schematic diagram, and designated by thegeneral reference character 704. Passgate circuit 704 may receive aninput signal (IN) on an input node 710 from a signal source 728 and passsuch a signal to an output node 708. Such signals may vary between highand low logic levels (VHI and VLO). Passgate circuit 704 may include,first and second transistors 726-0 and 726-1 of different conductivitytypes having source-drain paths arranged in parallel to one anotherbetween an input node 710 and an output node 708. Transistors 726-0 and726-1 may receive complementary enable signals EN and /EN at theirgates, respectively.

It is understood that either (or both) transistors may be DDCtransistors. Further, either or both transistors (regardless of whetherthey DDC transistors or not), may have bodies driven by high or lowlogic levels (VHI or VLO), or bodies static or dynamically biased todifferent voltages.

Embodiment may also include passgate and logic combinations. One suchparticular embodiment is shown in FIG. 8.

Referring now to FIG. 8, one example of a flip-flop circuit according toan embodiment is shown in a schematic diagram, and designated by thegeneral reference character 804. Passgate circuit 804 may include twostages 804-0/1, each of which includes an input passgate 830-0 andclocked latch formed by a latch passgate 830-1 and cross coupled latchinverters 832-0/1. Stages 804-0/1 may be enabled on complementary clockduty cycles. That is, as data is input to stage 804-0, data may belatched in stage 804-1 (e.g., CLK low), and as data is latched in stage804-0, it may be input to stage 804-1 (e.g., CLK high).

Any of passgates 830-0/1 may take the form of those passgate embodimentsshown herein, or equivalents. Similarly, any of inverters 830-0/1 maytake the form inverter embodiment shown herein, or equivalents.Accordingly, any or both of passgates 830-0/1 may include DDCtransistors, or may have separately biased bodies. Further, any or allof inverters 830-0/1 may include one or more DDC transistors, with anysuch DDC transistor having a bodies tied to a logic level, or biased tosome other voltage.

Embodiments like those of FIGS. 3A to 4B have shown “static” logiccircuit in which an output value and timing may be established by inputsignals. Alternate embodiments may include “dynamic” logic approaches.In a dynamic logic embodiment, an output logic level may be determinedaccording to input signals, however a timing of such an output signalmay be established by one or more timing signals. Particular dynamiclogic circuits embodiments are shown in FIGS. 9 to 10B.

Referring now to FIG. 9, one example of a dynamic logic circuitaccording to an embodiment is shown in a schematic diagram, anddesignated by the general reference character 904. A dynamic logiccircuit 904 may include a precharge transistor 936, an evaluationtransistor 938, and a logic section 934. A precharge transistor 936 mayconnect a precharge node 940 to a logic high level (VHI) in response toa timing signal (P/E) being low. An evaluation transistor 938 mayconnect a discharge node 942 to a low logic level (VLO) in response tothe timing signal (P/E) being high. A logic section 934 may receive oneor more input signals (IN0 to INj) and provide one or more outputsignals (OUT0 to OUTk). Input signals (IN0 to INj) may determine a stateof any output signals (OUT0 to OUTk). However the timing of such adetermination may be controlled according to timing signal (PIE).

While FIG. 9 shows precharge and evaluation transistors (936 and 938)being enabled/disabled in response to the same timing signal P/E,alternate embodiments may utilize separate signals to enable and disablesuch devices.

In some embodiments, a precharge transistor 936, an evaluationtransistor 938, or both, may be DDC transistors. In addition oralternatively, a logic section 934 may include one or more DDCtransistors. As in other embodiments above, DDC transistors may havelogic level tied bodies, or bodies statically or dynamically biased toother levels.

Referring now to FIGS. 10A and 10B, logic sections according toparticular embodiments that may included in a dynamic logic circuit,like that of FIG. 9, are shown in schematic diagrams.

Referring to FIG. 10A, a logic section 1034 may include a number oflogic transistors 1040-0 to 1040-2 that can selectively connect anoutput node 1008 to a precharge node 940 or a discharge node 942 inresponse to input values (IN0 to IN2). Any or all of such transistorsmay be DDC transistors. In the embodiment of FIG. 10A, all logictransistors 1040-0 to -2 may have source-drain paths that are connectedto a logic high or low level through a corresponding precharge orevaluation device (not shown in FIG. 10A). In the very particularembodiment shown, logic section 1034 may provide a logic function wherean output signal OUT0=inverse [IN0+IN1+IN2].

Referring to FIG. 10B, a logic section 1034′ may include an evaluationsection 1038 and an output inverter 1036. An evaluation section 1038 canselectively connect an input of inverter 1036 a precharge node 940 or adischarge node 942 in response to input values (IN0 to IN2), andaccording to a timing established by corresponding precharge orevaluation devices (not shown in FIG. 10B). Any or all the transistorswithin evaluation section 1038 may be DDC transistors. Output inverter1036 may drive output node 1008′ according to an output of evaluationsection 1038. In the very particular embodiment shown, logic section1034′ may provide a logic function where an output signalOUT0=IN0*IN2+IN1.

One skilled in the art could arrive at various other logic functionsaccording to teachings set forth.

In this way, dynamic logic circuits may include one or more DDC devices.

From the above examples, one skilled in the art would recognize thatdigital circuits according to the embodiments may include logic circuitconventions beyond static and dynamic approaches. As but one example,other embodiments may include “current steering” logic approaches. In acurrent steering embodiment, an output logic level may be determined bysteering current from two current paths according to received inputsignals. Particular current steering logic circuit embodiments are shownin FIGS. 11 to 12B.

Referring now to FIG. 11, one example of a current steering circuitaccording to an embodiment is shown in a block schematic diagram, anddesignated by the general reference character 1104. A current steeringlogic circuit 1104 may include respective output nodes 1108-0 and1108-1, a first current source 1144-0, a second current source 1144-1, acurrent sink 1146, and a logic section 1134. First and second currentsources 1144-0/1 may be connected in parallel between a logic high nodeVHI and a logic section 1134. First current source 1144-0 may provide acurrent to logic section 1134 via a first input current path 1148-0, andsecond current source 1144-1 may provide a current to logic section 1134via a second input current path 1148-1. According to input values (inthis embodiment, IN0 to INj and their complements), a logic section 1134may steer current from either input current path 1148-0/1 to currentsink 1146, and thus generate complementary output signals on suchcurrent paths 1148-0/1. A current sink 1146 may provide a current pathto a low voltage node VLO through sink current path 1150.

In the particular embodiment of FIG. 11, first and second currentsources 1144-0/1 may be p-channel transistors, which in particularembodiments may be DDC transistors. Similarly, current sink 1146 may bean re-channel transistor, which may in particular embodiments, be a DDCtransistor. As in other embodiments above, such DDC transistors may havebodies tied to logic levels, or have bodies driven by another voltage,either statically or dynamically.

Referring now to FIGS. 12A and 12B, logic sections according toparticular embodiments that may be included in a current steering logiccircuit, like that of FIG. 11, are shown in schematic diagrams.

Referring to FIG. 12A, a logic section 1234 may include logictransistors 1240-0 and 1240-1 that can selectively steer either of inputcurrent paths 1148-0/1 to current sink path 1150 according to an inputvalues (IN0 and its complement). When current is steered down one pathand not the other, complementary output values may be generated at inputcurrent paths 1148-0/1. Any or all of such transistors may be DDCtransistors subject to the various body biasing configurations notedherein. In the embodiment of FIG. 12A, logic section 1234 may provide alogic function of a buffer or an inverter, depending upon which inputvalues and output nodes are considered.

Referring to FIG. 12B, a logic section 1234′ may include a more complexarrangement of logic transistors 1240-2 to 1240-5. However, operationsmay occur in the same generation fashion as FIG. 12A, with suchtransistors (1240-2 to 1240-5) selectively steering current from one ofsteering current paths 1148-0/1 to current sink path 1150, to generatecomplementary output values at input current paths 1148-0/1. Any or allof such transistors may be DDC transistors subject to the various bodybiasing configurations noted herein. In the embodiment of FIG. 12B,logic section 1234′ may provide a NAND, NOR, AND or OR functiondepending upon which input values and output nodes are considered.

In this way, current steering logic circuits may include one or more DDCdevices.

While the flip-flop embodiments shown above may store data values,embodiments may include more compact digital data storage circuits. Inparticular, embodiments may include latches, and in particularembodiments, latches and memory cells with symmetrical matching devices.

Referring now to FIG. 13, a latch according to an embodiment is shown ina schematic diagram and designated by the general reference character1300. A latch 1300 may include driver transistors 1354-0/1 and loaddevices 1356-0/1. Driver transistors 1354- 0/1 may be cross-coupledbetween complementary data nodes 1352-0/1, having source-drain pathsconnected to a first logic level (in this case VLO), with a gate of onetransistor being connected to the drain of the other. Load devices1356-0/1 may be passive or active devices, connected in parallel betweendata nodes 1352-0/1 and second logic level (in this case VHI).

Driver transistors 1354-0/1 may be DDC transistors, and in particularembodiments, matching DDC transistors. DDC driver transistors may havebodies driven to logic levels, or to some other bias voltage,dynamically and/or statically.

A latch 1300 may store a data value on complementary data nodes1352-0/1, and may form part of various memory cell types, including butnot limited to four transistor (4T), 6T, and 8T static random accessmemory (SRAM) cells, to name but a few. Further, while FIG. 13 shows alatch with n-channel driver transistors, alternate embodiments mayinclude p-channel driver transistors.

In this way, a latch circuit may include DDC driver devices.

Referring now to FIG. 14, a 6-T SRAM cell according to one embodiment isshown in schematic diagram and designated by the general referencecharacter 1400. An SRAM cell 1400 may include items like those shown inFIG. 13, and such like items are referred to by the same referencecharacter.

FIG. 14 differs from FIG. 13 in that load devices may be p-channeltransistors 1456-0/1 cross coupled to one another. Further, n-channelaccess transistors 1458-0 and 1458-1 may connect bit lines 1460-0 and1460-1 to data nodes 1352-0 and 1352-1, respectively. In the embodimentof FIG. 14, all transistors may have bodies connected to a logic high orlogic low level, according to conductivity type. Further all transistorsmay be DDC transistors. Still further, transistors may match one anotherin a symmetrical fashion. That is, transistors 1456-0, 1458-0, and1354-0 may be the same size as, and fabricated in the same fashion ascorresponding transistors 1456-1, 1458-1, and 1354-1 respectively.

As will be described in more detail below, DDC transistors, by employinga substantially undoped channel, may provide less threshold variationthan conventional transistors, as such channels are less (or not)susceptible to random doping fluctuation (RDF). Consequently, asymmetrical latching structure may provide performance advantages overconventional latch circuits having doped channels subject to RDF.

Referring now to FIGS. 15A and 15B, a response of a 6-T SRAM cellaccording to an embodiment is shown. FIG. 15A is a schematic diagramshowing a 6-T cell like that of FIG. 14 under static noise margin (SNM)simulation conditions. Transistors in such a 6-T SRAM cell may have gatelengths of 28 nm. In the particular conditions shown, bit lines may heldat 0.7 volts, access devices may be driven to 1.0 V, and a high voltage(VHI) is 1.0 V, while a low voltage (VLO) is 0 V. A sweeping voltage(Vsrc) is applied between VLO and a data storage node that transitionsfrom 0 V to 1.0 V, and then back again.

FIG. 15B is a graph showing a response of the 6-T SRAM cell of FIG. 15under the noted simulation conditions. FIG. 15B shows two responsevariation ranges 1564 and 1562 showing responses to different sweepdirections of Vsrc. As shown, responses 1564 and 1562 include “eye”regions 1566-0 and 1566-1 that indicate stable switching between states.

Referring now to FIGS. 16A and 16B, a response of a 6-T SRAM cell isshown. FIG. 16A is a schematic diagram showing a 6-T cell that, unlikethe embodiment of FIGS. 15A and 15B, includes transistors with dopedchannels. Such doped channels are subject to RDF, as noted above,resulting in greater response variation. The conventional 6-T SRAM cellis subject to the same simulation conditions as FIG. 15A, and thetransistors may also have gate lengths of 28 nm.

FIG. 16B is a graph showing a response of a conventional 6-T SRAM cellunder the same simulation conditions as FIG. 15B. Like FIG. 15B, FIG.16B shows two response variation ranges 1664 and 1662 to the sweeping ofVsrc. However, due to RDF, resulting threshold voltage variationstranslate into a wider range of responses, resulting eye regions 1666-0and 1666-1 substantially smaller than those of FIG. 15B.

As noted in conjunction with FIG. 2A, a DDC transistor may take variousforms. A DDC transistor according to one very particular embodiment willnow be described with reference to FIG. 17. Such a transistor may beincluded in any of the embodiments shown above, or equivalents.

Referring to FIG. 17, a DDC according to a very particular embodiment isshown in a side cross sectional view.

As shown in FIG. 17, a DDC transistor 1760 may include a gate 1712separated from a substrate 1724 by a gate insulator 1722. A gate 1712may include insulating sidewalls 1768 formed on its sides. Source anddrain regions may include a lightly doped drain (LDD) structures 1776formed over deep source/drain diffusions 1774 to extend towards eachother under a portion of the gate. A DDC stacked channel structure maybe formed by a substantially undoped channel layer 1714, a thresholdvoltage (Vt) set layer 1770 formed by epitaxial growth and implant, oralternatively, by controlled out-diffusion from a screening layer 1716positioned below the undoped channel layer 1714. The screening layer1716 acts to define termination of the depletion zone below the gate,while the Vt set layer 1770 adjusts Vt to meet transistor designspecifications. In the embodiment shown, screening layer 1716 may beimplanted into body/bulk region 1718 so that it extends between and incontact with the source and drain diffusions 1774.

In a very particular embodiment, a DDC transistor 1760 may be ann-channel transistor having a gate length 1778 of 28 nm or less. Thescreening layer 1716 may have a carrier concentration of greater thanabout 5×10¹⁸ donors/cm³, while an overlying Vt set layer 1770 may have acarrier concentration of about 5×10¹⁷ to about 5×10¹⁸ donors/cm³. Asubstantially undoped channel region 1714 may have a carrierconcentration of less than about 5×10¹⁷ donors/cm³. It is understoodthat the above noted carrier concentrations are provided by way ofexample only and alternate embodiments may include differentconcentrations according to desired performance in a digital circuit.

A DDC transistor according to a further embodiment is shown in FIG. 18,and designated by the general reference character 1860. A DDC transistor1860 may include items like those shown in FIG. 17B, and like items arereferred to by the same reference character. DDC transistor 1860 differsfrom that of FIG. 17B in that screening layer 1716 may be implanted intobody/bulk region 1718 so that it extends below the gate withoutcontacting the source and drain diffusions 1774. The above DDCtransistors are but particular implementations of a DDC transistor, andshould not construed as unduly limiting the circuit elements includedwithin the various digital circuit embodiments shown herein.

As noted above, some embodiments may include DDC transistors andconventional doped channel transistors. FIG. 19 shows a portion of an ICsubstrate containing such two different types of transistors.

Referring to FIG. 19, an IC device 1900 may include DDC transistoractive regions (one shown as 1982-0) and conventional transistor activeregions (one shown as 1982-1) formed in a substrate and separated fromone another by isolation structure 1972. A DDC transistor active region1982-0 may include stacked channel structures formed below a controlgate, as described herein, to form one or more DDC transistors (oneshown as 1906). A conventional transistor active region 1982-1 mayinclude a doped channel formed below a control gate to form one or moreconventional transistors (one shown as 1980). Both types of transistors(e.g., 1982-0 and 1982-1) may form all or part of a digital circuit asdescribed herein, or equivalents.

In this way, an IC device having digital circuits may include both DDCtransistors and non-DDC transistors. Alternatively, selective masking toblock out areas of a die for manufacture of DDC or non-DDC transistorscan be employed, or any other conventional technique for manufacturingdie having at least some DDC transistors. This is particularly usefulfor mixed signal die having multiple transistors types, including highspeed digital logic and analog transistors, as well as power efficientlogic and/or memory transistors.

Digital circuits according to embodiments shown herein, and equivalents,may provide improved performance over conventional circuits by operatingwith transistors (e.g., DDC transistors) having lower threshold voltage(Vt) variability. Possible improvements may include faster signalpropagation times, as noted above.

Improved performance may translate into reductions in device size. Asbut one example, digital circuit transistors may be sized with respectto one another to achieve a particular response. Such sizing may takeinto account expected variations in Vts. Because DDC transistors havelower Vt variation, less sizing margin may be necessary to achieve adesired response. As but one very particular example, SRAM cells mayhave a predetermined sizing between access transistors and drivertransistors. SRAM cells according to the embodiments may lower arelative size scaling between such devices, relative to comparably sizedconventional transistors. As SRAM cells may be repeated thousands, oreven millions of times in a device, reductions in size by extend beyondexpected limits presented by conventional arrays incorporating SRAMcells with doped channels.

In addition, such improvements may include lower operating voltages. Inthe embodiments, digital circuit switching voltages, established bytransistor Vts, may be subject to less variability. Accordingly, a“worst” switching point may be lower, allowing for an operating voltageto be correspondingly lower. In some embodiments, operating voltages(Vsupply) may be no greater than 1 V, and a threshold voltage may be nogreater than 0.6*Vsupply.

As noted above, in some embodiments digital circuits may include DDCtransistors body bias connections driven with a bias voltage differentthan a logic high or low voltage. A screening layer within suchtransistors may enable higher body effect for modulating thresholdvoltage. In such embodiments, a variation in threshold utilizing a bodyeffect may be achieved with a lower body bias voltage than conventionaltransistors. Body effect modulation may enable bodies to be driven toreduce threshold voltage, and hence reduce leakage.

Digital circuits according to embodiments may have lower powerconsumption than circuits employing conventional doped channels. Asnoted above, because a worst case threshold voltage variation may below, a power supply voltage may be reduced, which may reduce powerconsumption. In addition, substantially undoped channels in DDC devicesmay have improved mobility as compared to some conventional transistors,and hence provide lower channel resistance.

It should be appreciated that in the foregoing description of exemplaryembodiments of the invention, various features of the invention aresometimes grouped together in a single embodiment, figure, ordescription thereof for the purpose of streamlining the disclosureaiding in the understanding of one or more of the various inventiveaspects. This method of disclosure, however, is not to be interpreted asreflecting an intention that the claimed invention requires morefeatures than are expressly recited in each claim. Rather, as thefollowing claims reflect, inventive aspects lie in less than allfeatures of a single foregoing disclosed embodiment. Thus, the claimsfollowing the detailed description are hereby expressly incorporatedinto this detailed description, with each claim standing on its own as aseparate embodiment of this invention.

It is also understood that the embodiments of the invention may bepracticed in the absence of an element and/or step not specificallydisclosed. That is, an inventive feature of the invention may beelimination of an element.

Accordingly, while the various aspects of the particular embodiments setforth herein have been described in detail, the present invention couldbe subject to various changes, substitutions, and alterations withoutdeparting from the spirit and scope of the invention.

1-17. (canceled)
 18. A digital logic circuit, comprising: a plurality ofMOSFET transistors constituting a current steering logic circuitincluding respective first and second output nodes, a first currentsource, a second current source, a current sink, and a logic section;wherein the first and the second current sources are connected inparallel between a logic high node and the logic section, the firstcurrent source provides a current to the logic section via a first inputcurrent path, the second current source provides a current to the logicsection via a second input current path; wherein the logic sectionsteers current from either of the first and the second input currentpath to the current sink according to input values of the logic section,and thus generates complementary output signals; and wherein each of theplurality of transistors has a deeply depleted channel formed below itsgate that includes a substantially undoped channel region formed fullyover a relatively highly doped screening layer formed fully over and incontact with a doped body region.